Solution polymerization processes are commercially used to prepare a wide variety of ethylene polymers, ranging from crystalline polyethylene plastics to amorphous ethylene-propylene elastomers. It is desirable to operate these processes at high temperatures because increasing the polymerization temperature can (a) improve the rate of polymerization; (b) facilitate the removal of the enthalpy of polymerization (i.e. reactor cooling systems generally become more thermodynamically efficient as the temperature gradient between the reactor and the cooling system is increased); (c) lower the viscosity of the polymer solution; and (d) reduce the amount of energy required to recover the polymer from the solvent. Such solution polymerization reactions often employ a catalyst system which contains a group 4 or group 5 metal, especially titanium and/or vanadium. The catalysts may be comparatively simple transition metal molecules (especially transition metal halides or alkoxides which are used in the so-called Ziegler-Natta catalyst systems) or comparatively more complex mono or bis cyclopentadienyl organometallic molecules which are generally referred to as "metallocenes".
Metallocenes are the catalysts of choice when it is desired to produce ethylene copolymers having uniform comonomer incorporation and a narrow molecular weight distribution. However, most metallocene catalysts are quite temperature sensitive (i.e. the molecular weight of the polymers produced with metallocene catalysts tends to drop to undesirable low values as the temperature is increased under solution polymerization conditions). It is generally theorized that this temperature dependency is a function of a temperature-sensitive beta hydride elimination reaction (and evidence of this is found in the observation that many metallocene catalysts function very well at high temperatures if high ethylene pressures are also used--as in a high pressure or bulk polymerization). However, the maximum ethylene concentration available to the catalyst in a solution polymerization is limited by the solubility of ethylene in the solvent--with the result that many metallocene catalysts are not suitable for use in a solution process. Simply put, many metallocene catalysts don't do the job at the preferred (high) operating temperatures for a solution process.
This problem can sometimes be mitigated using a metallocene catalyst having a bridged ligand--especially those catalysts which incorporate the so-called Bercaw ligand (also known as "constrained geometry" catalysts--see U.S. Pat. No. ("USP") 5,055,438 to Canich and U.S. Pat. No. 5,350,723 to Neithamer et al. However, bridged ligands are difficult and expensive to synthesize. Accordingly, there is a need for comparatively simple unbridged metallocene catalysts for solution polymerizations.
Unbridged catalysts having one cyclopentadienyl ligand and one nitrogen-containing ligand have been disclosed in the art. For example, U.S. Pat. No. 5,625,016 (Schiffino et al) addresses this need and teaches a high temperature solution polymerization process which employs a catalyst that is an unbridged group 4 metal complex having a bulky cyclopentadienyl ligand and a bulky group 15 heteroatom ligand. The use of the bulky cyclopentadienyl ligand is essential to the Schiffino et al catalyst. [Schiffino et al note that Japanese Kokai 94/80683 ("JP '683") discloses a propylene polymerization catalyst having a non-bulky cyclopentadienyl ligand and a bulky group 15 heteroatom ligand. Schiffino et al provide experimental data which clearly illustrate that a catalyst of the JP '683 reference is not a suitable catalyst for the high temperature solution copolymerization of ethylene. However, Schiffino et al provide inventive data which illustrates that the use of a bulky cyclopentadienyl ligand does produce a useful catalyst.]
The group 15 heteroatom ligand initially disclosed in the JP '683 reference (and subsequently employed by Schiffino et al) is characterized by having a nitrogen atom which is bonded to the transition metal and substituted with two bulky substituents (in particular, two bulky trimethyl silyl groups).
In a copending and commonly assigned application (Stephan et al) there is disclosed a solution polymerization catalyst having a cyclopentadienyl ligand and a phosphinimine ligand. The phosphinimine ligand has a nitrogen atom which is bonded to the transition metal and doubly bonded to a phosphorous (v) atom. Thus, there is only one substituent on the nitrogen atom of the phosphinimine ligands disclosed by Stephan et al (namely the phosphorous (v) atom) whereas the heteroatom ligands taught by Schiffino et al and JP '683 have two substituents on the nitrogen ligand.
Schiffino et al and Stephan et al both disclose the use of two alternative activators, namely (1) alumoxanes; and (2) "ionic activators". Alumoxanes were discovered to be excellent activators for metallocenes by Kaminsky and Sinn, as claimed in U.S. Pat. No. 4,404,344. Hlatky and Turner subsequently discovered that ionic activators function well with bis(cyclopentadienyl) metallocene complexes (see for example, U.S. Pat. No. 5,198,401).
Although the Schiffino et al and Stephan et al references described above do disclose catalysts having utility in solution polymerizations, there is still a need for other simple, robust catalysts which function well in solution polymerizations.